The term “opiate” has been used to designate pharmacologically active alkaloids derived from opium, e.g., morphine, codeine, and many semi-synthetic congeners of morphine. After the isolation of peptide compounds with morphine-like actions, the term opioid was introduced to refer generically to all drugs with morphine-like actions. Included among opioids are various peptides that exhibit morphine-like activity, such as endorphins, enkephalins and dynorphins. However, some sources use the term “opiate” in a generic sense, and in such contexts, opiate and opioid are interchangeable. Additionally, the term opioid has been used to refer to antagonists of morphine-like drugs as well as to characterize receptors or binding sites that combine with such agents.
Opioids are generally employed as analgesics, but they may have many other pharmacological effects as well. Morphine and related opioids produce certain of their major effects on the central nervous and digestive systems. The effects are diverse, including analgesia, drowsiness, mood changes, respiratory depression, dizziness, mental clouding, dysphoria, pruritus, increased pressure in the biliary tract, decreased gastrointestinal motility, nausea, vomiting, and alterations of the endocrine and autonomic nervous systems.
When therapeutic doses of morphine are given to patients with pain, they report that the pain is less intense, less discomforting, or entirely gone. In addition to experiencing relief of distress, some patients experience euphoria. However, when morphine in a selected pain-relieving dose is given to a pain-free individual, the experience is not always pleasant; nausea is common, and vomiting may also occur. Drowsiness, inability to concentrate, difficulty in mentation, apathy, lessened physical activity, reduced visual acuity, and lethargy may ensue.
Two distinct classes of opioid molecules can bind opioid receptors: the opioid peptides (e.g., the enkephalins, dynorphins, and endorphins) and the alkaloid opiates (e.g., morphine, etorphine, diprenorphine and naloxone). Subsequent to the initial demonstration of opiate binding sites (Pert, C. B. and Snyder, S. H., Science (1973) 179:1011-1014), the differential pharmacological and physiological effects of both opioid peptide analogues and alkaloid opiates served to delineate multiple opioid receptors. Accordingly, three molecularly and pharmacologically distinct opioid receptor types have been described: delta, kappa and mu. Furthermore, each type is believed to have sub-types (Wollemann, M., J Neurochem (1990) 54:1095-1101; Lord, J. A., et al., Nature (1977) 267:495-499).
All three of these opioid receptor types appear to share the same functional mechanisms at a cellular level. For example, the opioid receptors cause inhibition of adenylate cyclase, and inhibition of neurotransmitter release via both potassium channel activation and inhibition of Ca2+ channels (Evans, C. J., In: Biological Basis of Substance Abuse, S. G. Korenman & J. D. Barchas, eds., Oxford University Press (in press); North, A. R., et al., Proc Natl Acad Sci USA (1990) 87:7025-29; Gross, R. A., et al., Proc Natl Acad Sci USA (1990) 87:7025-29; Sharma, S. K., et al., Proc Natl Acad Sci USA (1975) 72:3092-96). Although the functional mechanisms are the same, the behavioral manifestations of receptor-selective drugs differ greatly (Gilbert, P. E. & Martin, W. R., J Pharmacol Exp Ther (1976) 198:66-82). Such differences may be attributable in part to the anatomical location of the different receptors.
Delta receptors have a more discrete distribution within the mammalian CNS than either mu or kappa receptors, with high concentrations in the amygdaloid complex, striatum, substantia nigra, olfactory bulb, olfactory tubercles, hippocampal formation, and the cerebral cortex (Mansour, A., et al., Trends in Neurosci (1988) 11:308-14). The rat cerebellum is remarkably devoid of opioid receptors including delta opioid receptors.
There is a continuing need for new delta opioid receptor modulators as analgesics. There is a further need for delta opioid receptor selective agonists as analgesics having reduced side effects. There is also a need for delta opioid receptor antagonists as immunosuppressants, antiinflammatory agents, agents for the treatment of neurological and psychiatric conditions, agents for the treatment of urological and reproductive conditions, medicaments for drug and alcohol abuse, agents for treating gastritis and diarrhea, cardiovascular agents and agents for the treatment of respiratory diseases, having reduced side effects.
There is a continuing need for new opioid receptor modulators as analgesics. There is a further need for delta and mu opioid receptor agonists as analgesics having reduced side effects. There is a further need for mu opioid receptor agonists as analgesics having reduced side effects for the treatment of pain, immune function, esophageal reflux, and cough. There is also a need for delta opioid receptor agonists as analgesic agents, agents for the treatment of respiratory diseases, cardiovascular agents, agents for treating urological dysfunction, and agents for the treatment of neurological and psychiatric conditions. There is further need for dual delta opioid receptor/mu opioid receptor agonists.